Firewire, only interesting if you have firewire devices. If you don't and get one later, you can buy a fire wire expansion card for about $15. So to get the + version isn't nessesary if you don't want firewire.
Alot of good boards feature SLi, but that's not why to get them. If you should want sli, you would have to get very extreme cards, otherwise it's smarter to get one good card. Then later on when you have more money, sell the old grapics card an update to a newer version. SLi, you want the fastest board out the and double it. Otherwise the sli solution will perform a little bit worse than a singel card equal in the prise. So go with a sli motherboard if you like the other features on it aswell, otherwise it's not that important.
I must recomend the 6800GS though, my friend has one at stock speeds. It scores equally as my $650 6800 Ultra used to do, for about $275.
But if you manage to get just another $25-50 budget, get a 7800GT. Or wait a while and get 7900GT or wait until prices drop on x1900xt.